Warn if building a library with explicit output file name

[catamorphism]

Warn if rustc is building a library and the command-line also contained a -o flag, since the library file name will be the same regardless. (See a FIXME in driver::driver::build_output_filenames.)

[bstrie]

This:

$ rustc --lib lauxlibh_lua52.rs
warning: missing crate link meta `name`, using `lauxlibh_lua52` as default
warning: missing crate link meta `vers`, using `0.0` as default

produces liblauxlibh_lua52-68a2c114141ca-0.0.so, while this:

$ rustc --lib -o asdf.so lauxlibh_lua52.rs
warning: missing crate link meta `name`, using `asdf` as default
warning: missing crate link meta `vers`, using `0.0` as default

produces libasdf-68a2c114141ca-0.0.so.

So it appears that -o now affects the name of the output lib.